1. Field of the Invention
The present invention relates particularly to an image forming apparatus using an electron source.
2. Description of the Related Art
Hitherto, there are known two types of electron emitting devices, i.e., a thermionic cathode and a cold cathode. Of these two types, known examples of the cold cathode include a surface conductive type electron emitting device, a field emission type electron emitting device (referred to as “FE type” hereinafter), and a metal/insulator/metal type electron emitting device (referred to as “MIM type” hereinafter).
Some examples of the surface conductive type electron emitting devices are described in M. I. Elinson, Radio Eng. Electron Phys., 10, 1290(1965) and other papers mentioned below.
A surface conductive type electron emitting device utilizes a phenomenon that electron emission occurs when an electric current is supplied to a small-area thin film formed on a substrate so as to flow parallel to the film surface. Surface conductive type electron emitting devices known so far employ an SnO2 thin film, as reported by M. I. Elinson et al., an Au thin film [see, e.g., G. Dittmer: “Thin Solid Films”, 9, 317(1972)], an In2O3/SnO2 thin film [see, e.g., M. Hartwell and G. G. Fonstad: “IEEE Trans. ED conf.”, 519(1975)], a carbon thin film [see, e.g., Hisashi Araki et al.: Shinku (Vacuum), vol. 26, No. 1, 22(1983)], etc.
As a typical example of one of those surface conductive type electron emitting devices, FIG. 12 shows a plan view of the device reported by M. Hartwell et al.
Referring to FIG. 12, numeral 3001 denotes a substrate and 3004 denotes a conductive thin film of a metal oxide formed by sputtering. As shown, the conductive thin film 3004 is formed into an H-shape as viewed from above. An electron emitting portion 3005 is formed by carrying out an energization process to be described later, called “energization forming”, on the conductive thin film 3004. A spacing L shown in FIG. 12 is set to 0.5-1 mm and a width W is set to 0.1 mm. Note that although the electron emitting portion 3005 is shown as having a rectangular shape at the center of the conductive thin film 3004, the drawing has been illustrated for the sake of easier understanding and does not exactly express the exact position and shape of electron emitting portions actually physically produced.
Known FE type electron emitting devices are reported, for example, by W. P. Dyke & W. W. Dolan, “Field Emission”, Advance in Electron Physics, 8, 89(1956) and C. A. Spindt, “Physical properties of thin-film field emission cathodes with molybdenum cones”, J. Appl. Phys., 47, 5248(1976).
As a typical example of a construction of a FE type electron emitting device, FIG. 13 shows a sectional view of the device reported by C. A. Spindt et al.
Referring to FIG. 13, numeral 3010 denotes a substrate, and 3011 denotes an emitter wire made of a conductive material. Numeral 3012 denotes an emitter cone, 3013 denotes an insulating layer, and 3014 denotes a gate electrode. In the FE type device, field emission occurs from the top of the emitter cone 3012 by applying an appropriate voltage between the emitter cone 3012 and the gate electrode 3014.
As another example of a FE type device construction, there also is known a planar structure wherein an emitter and a gate electrode are arranged on a substrate, and lay substantially parallel to a flat surface of the substrate, rather than as shown in FIG. 13.
A known MIM type electron emitting device is reported, for example, by C. A. Mead, “Operation of Tunnel-emission Devices”, J. Appl. Phys., 32, 646(1961).
A typical example of a construction of the MIM type electron emitting device is shown in a sectional view of FIG. 14. Referring to FIG. 14, numeral 3020 denotes a substrate, and 3021 denotes a metal lower electrode. Numeral 3022 denotes a thin insulating layer having a thickness of about 10 nm, and 3023 denotes a metal upper electrode having a thickness of about 8-30 nm. In the MIM type device, electron emission occurs from the surface of the upper electrode 3023 by applying an appropriate voltage between the upper electrode 3023 and the lower electrode 3021.
Any of the cold cathodes described above do not require a heater for heating the devices because the cold cathodes can produce an electron emission at a lower temperature than, that needed in the thermionic cathode. Therefore, a cold cathode can be formed with a simpler structure and a finer pattern than a thermionic cathode. Also, when a large number of cathodes are arrayed on a substrate with a high density, a problem such as thermal fusion of the substrate is less likely to occur. Further, a cold cathode has a high response speed, whereas a thermionic cathode has a low response speed because it starts operation upon heating by the heater.
For those reasons, studies regarding applications of cold cathodes have been actively conducted.
As to applications of the electron emitting devices, image forming apparatuses such as an image display unit and an image recording apparatus, charged beam sources, etc., have been studied.
Applications of the electron emitting devices to image forming apparatuses are disclosed in, for example, U.S. Pat. Nos. 5,532,548, 5,770,918 and 5,903,108, WO Nos. 98/28774 and 99/03126, as well as Japanese Patent Laid-Open Nos. 01-241742, 04-094038, 04-098744, 04-163833 and 04-284340.
Of those image forming apparatuses employing the electron emitting devices, attention often is focused on a flat display which has a thin body contributing to saving space, and which also is lightweight and expected to be eventually substituted for a CRT type display.
FIG. 20 is a perspective view schematically showing a partially uncovered flat image forming apparatus (airtight container) that employs an electron source comprising a number of electron emitting devices arrayed in the form of a matrix. In FIG. 20, numeral 27 denotes an electron emitting device of any type described above, and numerals 23 and 24 denote wires connected to the electron emitting device 27. Numeral 1 denotes a rear plate on which the electron emitting devices are arrayed, 20 denotes an image forming member made up of a phosphor, etc., and 19 denotes a metal film (metal back) to which a high voltage (Hv) is applied for irradiating electrons emitted from the electron emitting devices towards the image forming member. Numeral 11 denotes a face plate on one side of which the image forming member is arranged, and 4 denotes a support frame which, together with the face plate 11 and the rear plate 1, constitutes an airtight container 100. An inner space of the airtight container 100 is held in a vacuum state at a level of about 10−4 Pa (Pascal).